Comparison of Soil Organic Carbon Methods.

Soil organic carbon (SOC) is an important property in regard to soil and environmental quality. There are many methods to determine SOC, but there are no previous studies that compare many methods using a large variety of soils. In this research, nine methods were chosen, six variables were evaluated, and more than 300 samples representing a high diversity of soils were used for SOC methods comparison. The nine methods included: dry combustion, Walkley-Black, loss on ignition, photometry, NIRS, Solvita, color, texture, and visualization. The six variables were: equipment, accuracy, efficiency, complexity, cost and environmental effect. More than 300 samples from different regions in the U.S. and other countries were analyzed. All methods were performed following standard protocols except for those specifically indicated. Deming regression was used to compare the accuracy of other methods with dry combustion. The results, using dry combustion as the standard, showed that Walkley-Black (r²=0.89), loss on ignition (r²=0.86), and Solvita (r²=0.64) have satisfactory correlations; photometry is a good semi-quantitative method with a modest accuracy (r²=0.31); methods like NIRS, color, texture, and visualization need further research and justification. Loss on ignition was the preferred alternative to dry combustion when taking all factors into consideration.